There exists a class of devices exemplified by U.S. Pat. No. 4,020,830 to Johnson et al., dated May 3, 1977, entitled SELECTIVE CHEMICAL SENSITIVE FET TRANSDUCERS, which features electrical characteristics modulated by the interaction of a chemically selective system to specified ambient materials. In accordance with the Johnson patent, a substrate layer carries respective drain and source regions, separated by a region or channel over which is disposed an insulating layer and a chemically selective system for specified interaction with predetermined ambient materials. The chemically selective system generally takes the form of a membrane which interacts with the materials, and modulates the drain to source electrical conduction based on concentrations of the specified ambient substances. The Johnson et al. patent contemplates chemically selective systems for measuring various types of ambient conditions, including gas concentrations, ion activity, immunochemical concentrations, concentrations of enzymes and substrates, and the like, and indeed many such applications have found favor in a variety of disparate fields. While the nomenclature in the art has tended to designate these respective applications separately, for example utilizing the designation CHEMFET for chemically selective membrane devices, ISFET for ionically reactive devices, IMMUNOFET for immunologically reactive devices, and so forth, for purposes of this application the term "chemfet", or simply "device" shall be utilized generically to encompass all such apparatus, irrespective of the type of sensing or reaction utilized, character of the membrane employed, or nature of the ambient substance to be monitored. Likewise, the terms chemfet or device as used herein shall embrace transistor-type, diode-type, or the like other devices which feature similar conductivity modulation based on membrane-substance interaction.
In recent times, much effort has been expended in the development of device configurations and manufacturing processes which will facilitate large-scale production of reliable, stable, and well-calibrated devices. For example, device encapsulation, membrane formulation, and membrane disposition have proven to be formidable technical problems.
It is a general object of the present invention to provide device configurations and manufacturing processes for the production of superior chemfet devices, which have high reliability, well quantified specifications, suitable physical integrity, and a reasonable operational lifespan. With particular reference to the membrane aspect of the devices, it is to be noted that problems have been encountered both in the fabrication phase, and in the use and lifespan aspects of the devices. For example, polymeric membranes with high plasticizer content have found favor in the field, but production yield, operational reproducability, and physical integrity have all too often characterized the devices. Obviously, a chemfet membrane which is inadequately adhered to the gate insulator at the time of production will result in, at best, a gradual detachment of the membrane from the surrounding encapsulation. This results not only in a progressive loss of chemical response, but sooner or later in total failure of the device.
Accordingly, more specific objects of the present invention relate to provision of mechanisms, systems, and techniques whereby chemfet membranes are reliably, certainly, and substantially permanently attached to the device, thereby achieving superior mechanical integrity, and improved, well-characterized electrochemical operation.
It is a further specific object of the present invention to provide methods and structures whereby the devices may be effectively manufactured and encapsulated (i.e. hermetically sealed except for the controlled exposure of the gate area), without undue damage either to the gate membrane area, or to the device at large.